1. Field of the Invention
The present invention relates to a communications device for a control device, which transmits digital information from a transmitter to the control device through a signal line.
2. Description of the Prior Art
A control system for an automatic transmission that has been practically used includes a control unit provided with a microcomputer circuit and exclusively used for controlling the automatic transmission. This control unit controls the operation of solenoid valves provided in the automatic transmission according to a predetermined processing program, so as to subtly control switching or shifting of gear positions of the transmission and setting of an oil pressure level used for shifting.
To the automatic transmission control unit, there are connected a plurality of sensors provided in the automatic transmission and engine of the vehicle. Further, the automatic transmission control unit receives digital information through signal lines from a number of control devices, such as an engine control unit for controlling the engine.
The automatic transmission control unit determines the optimum shifting time based on input information received from these sensors and surrounding equipment, and changes electric signals to be outputted to solenoids to thus cause the automatic transmission to perform a suitable shifting operation.
When the automatic transmission control unit receives digital information from switches, sensors and other control devices, the voltage of the signal line is normally switched between two levels that correspond to logical values 1/0 of the digital information.
Japanese laid-open Patent Publication No 1-172668 discloses a communications device for transmitting a digital signal from the automatic transmission control unit to the engine control unit. In this device, two microcomputer circuits (operation units of the automatic transmission control unit and engine control unit) communicate with each other through a signal line provided with a simple interface circuit, and the microcomputer circuit on the transmitter side forms a binary signal in the signal line, which signal is then directly read by the microcomputer circuit on the receiver side.
The microcomputer circuit on the transmitter side also detects the voltage level of the signal line outside the interface circuit, and compares the detected level with the output (1/0) of the transmitter-side microcomputer circuit, so as to determine an abnormality or failure of the signal line when the detected voltage level does not normally correspond to the output of the transmitter.
Where the voltage level of the signal line is simply switched between two levels corresponding to the logical values 1/0 of the digital information, the communications device as disclosed in Japanese laid-open Patent Publication No. 1-172668 only requires simple interface circuits on the transmitter side and receiver side, and is able to check the content of communication on the transmitter side. The receiver, however, is not able to determine whether the transmitted digital information is correct or not, based on the 1/0 digital information read from the signal line.
Namely, the receiver is not able to discriminate 0 V from the signal line short-circuited to the ground level, from L (low) level of a normal 1-bit signal, nor be able to discriminate an abnormal voltage level due to breakage of the interface circuit on the transmitter side or receiver side, from H (high) level of the normal 1-bit signal.
If a communications device is adapted to transmit digital information with 1/0 logical values corresponding to two voltage levels that are between the power supply voltage and the ground potential, the communication abnormality can be judged when a voltage outside these two voltages is detected in the signal line.
In this communications device, however, the transmitter requires an interface circuit for forming an analog voltage corresponding to the logical value 1/0 of the digital information, and the receiver requires an interface circuit for reading the digital information from the analog voltage and an abnormality detecting circuit for detecting an abnormality of the analog voltage.
In another communications device which is provided with two communication lines through which the same digital information is transmitted in parallel with each other, abnormality in the communication can be judged when the information transmitted through one of the lines is different from that transmitted through the other line.
In this case, however, each of the transmitter and receiver needs two sets of interface circuits, and the microcomputer circuit of the receiver needs to have two input ports. This is incompatible with recent requirements for reduced size of equipment and simplified wiring.
It is also possible to apply a method of so-called serial communication in which a synchronization signal of a given frequency is produced in a communication line, and a binary signal corresponding to the logical value 1/0 of the digital information is transmitted in association with the synchronization signal.
In this case, however, a special interface circuit and abnormality detecting circuit need to be additionally provided on the receiver side, as in the case where analog voltages are used. Since the synchronization signal does not match the frequency of program processing effected in the control device on the receiver side, the receiver-side control device cannot directly read the binary signal from the communication line. Accordingly, a read circuit (shift register) is needed which operates in synchronization with the synchronization signal to reproduce the digital information from the binary signal, and retain the information until the time when the receiver-side control device is ready to read the reproduced information. Further, an additional abnormality detecting circuit is needed for detecting that the synchronization signal ceases to be transmitted.